Processes for making reformed gases of various types are widely used throughout the world, and have particular application in connection with direct reduced iron (DRI) plants. In the DR process, the reformer is used to reform natural gas (CH4) with the oxidants CO2 and H2O from a recycled process gas obtained from a reduction furnace, also referred to as a shaft furnace, for example. The reductants CO and H2 are formed in the reforming reaction, which are then used at elevated temperature to reduce iron oxide (Fe2O3), i.e. iron ore, to metallic iron (Fe) in the reduction furnace. The metallic iron is subsequently processed into various steel grades for fabricating final products.
This DR process is illustrated generally in FIG. 1, and includes three major steps: reduction, reformation, and heat recovery. In the reduction step, the iron oxide, in pellet or lump form, is introduced at the top of the reduction furnace 10 through a proportioning hopper 12. As the iron oxide descends through the reduction furnace 10 by gravity flow, it is heated and the oxygen is removed from the iron, i.e. the iron oxide is reduced, by counter-flowing gases that have high contents of the reductants CO and H2. These gases react with the Fe2O3 in the iron ore and convert it to the metallic iron, leaving the oxidants CO2 and H2O. Accordingly, the reduction furnace 10 has three distinct zones in which the DR process is carried out: a reduction zone, a transition zone, and a cooling zone. For the production of cold DRI, the metallic iron is cooled and carburized by counter-flowing cooling gases in the lower portion of the reduction furnace 10. The DRI may also be discharged hot, and fed to a briquetting machine for the production of hot briquetted iron (HBI), or fed hot, as hot direct reduced iron (HDRI), directly to an electric arc furnace (EAF), etc.
In the reformation step, in order to maximize reforming efficiency, recycled process gas from the reduction furnace 10 is blended with fresh natural gas and fed to the reformer 14, a refractory lined furnace including one or more metallurgical alloy reformer tube apparatuses 16 filled with a catalyst, such as nickel or nickel alumina. The gas is heated and reformed as it passes through the reformer tube apparatuses 16. The newly reformed gas, containing 90-92% CO and H2, is then fed hot directly to the reduction furnace 10 as the reducing gas.
In the heat recovery step, the thermal efficiency of the reformer 14 is maximized. Heat is recovered from the reformer flue gas and used to preheat the reformer feed gas mixture, the burner combustion air, and the natural gas feed. Optionally, the reformer fuel gas is also preheated.
Since the presence of oxidants in the reformed gas would hinder the reduction reaction, the reformer feed gas mixture must contain sufficient oxidants to react with the natural gas, plus sufficient excess oxidants to protect the catalyst. This is referred to as stoichiometric reforming. The reductant-to-oxidant ratio in the reformed gas is typically about 11-to-1. The reforming reaction is endothermic. Thus, the input of heat is required for the reaction. The reforming reaction takes place in the presence of a catalyst to accelerate the reaction rate. Because one of the oxidants is CO2, the reformer 14 must be operated at higher temperatures than conventional steam reformers.
Conventional reformer tube apparatuses 16 are manufactured from various metallurgical alloys to design specifications that result in a life span of 7-10 years at controlled operating temperatures. A set of replacement tubes 16 may cost upwards of $10,000,000.00, representing a significant cost to the operator of a DRI plant, for example. Thus, it would be advantageous if the tubes 16 were capable of operating at current temperature levels for longer periods of time. Likewise, it would be advantageous if the tubes 16 were capable of operating at increased temperature levels for the same period of time. Both situations would provide an increase in the production output of the reformer 14, thereby providing an increase in the production output of the DRI plant, and, ultimately, profits.
Most conventional tubes 16 eventually fail at their top section, near the reformer roof. This localized section gradually creeps and grows in diameter, forming a “bulge.” This is an area of unwanted deformation and wall thinning. Conventional approaches to solving this problem include increasing the wall thickness of the entire tube 16, resulting in increased overall weight, less efficient heat transfer, support issues, and increased incidental tube stretching, all resulting in significant additional cost. A solution to this problem is needed, but has not been developed by those of ordinary skill in the art.